We shall isolate dimeric intermediates in the genetic recombination pathways of phage lambda. Dimers generated by Red and Rec pathways will be separately characterized. Dimers will be isolated by isopycnic centrifugation following infection by dense and radioactive particles in the absence of replication. Added measures to improve the frequency of recombination and to ensure the stability of intermediates will be taken by using suitable mutants and by cross linking the DNA prior to isolation. Concentration of the intermediates on a BND-cellulose column and their isolation as a DNA-protein complex will also be attempted. The isolated dimers will be defined topologically and topographically (following partial denaturation) by electron microscopy.